1. Field of the Invention
The invention concerns a camshaft for an internal combustion engine (ICE) having variable valve actuation. Specifically, the present invention concerns a camshaft wherein the amount of dwell varies with respect to the angular velocity of the ICE.
Varying dwell, or the duration a poppet valve is open, affects fuel economy at part load on the ICE (by varying the occurrence of inlet valve closure, i.e. earlier or later), low speed power output (by flattening the torque curve) and full throttle fuel economy (by varying inlet and outlet valve overlap), as well as reduces the emissions from the ICE.
Either early or late inlet valve closure can increase the efficiency of an ICE operating at part load. Reducing the overlap period of the inlet and outlet valves can improve part throttle performance of an ICE by minimizing interference between induction and exhaust cycles. The gains in performance are greatest while the ICE is at idle (i.e. when inlet manifold pressure is highest). On the other hand, increased valve overlap leads to reduced NOx emissions at higher loads on the ICE and increased hydrocarbon emissions at lower loads on the ICE.
Dwell is a function of the profile of a cam. A cam profile in which the cam maintains a valve in a fully lifted, i.e. open, position is considered to have a "long" dwell. Conversely, a cam profile which causes a valve to open and close within a relatively short period is considered to have a "short" dwell.
An engine with long dwell cams is generally characterized as demonstrating superior performance at relatively high angular velocities of the engine. Essentially, a long dwell camshaft eases engine breathing (i.e the ability to draw in a fresh charge of unburned fuel and air, as well as discharge spent combustion products). An engine with short dwell cams generally operates most efficiently at relatively low engine speeds where it is also desirable to limit the quantity of emissions such as carbon monoxide, unburned hydro-carbons, etc.
The primary disadvantage of conventional camshafts is the requirement of selecting a single cam profile which favors either high speed performance or low speed emissions. This disadvantage is particularly prominent in small displacement air-cooled engines which are generally operated for extended periods at both extremes.
Another aspect of the preset invention concerns the ability of a camshaft for an ICE to vary the relative timing between operating the intake and exhaust valves with respect to the angular velocity of the ICE. Relative valve timing according to the present invention may be accomplished either separately or in conjunction with the aforementioned variation in dwell.
2. Description of Related Art
A number of attempts to accomplish variable valve actuation have been attempted in the past.
Woydt et al. (U.S. Pat. No. 1,527,456) discloses a system for relatively rotating two adjacent cams actuating a common tappet. Disadvantages of the Woydt et al. arrangement are a dramatically weakened hollow cam member 21 (due to the presence of longitudinal slots 35), and the requirement for an external control system 26-29 to regulate operation of the system.
Hellmann (U.S. Pat. No. 2,888,837) discloses a system whereby an actuating rod 15 causes adjacent pairs of cooperating cams 7,8 to pivotally separate. Disadvantages of the Hellmann arrangement include the same weakness in the camshaft 1 and changes in the profile of the ramps up to the peaks of the cam lobes as the cams pivot with respect to the shaft.
Goodfellow et al. (U.S. Pat. No. 3,144,009) disclose a system similar to Woydt et al. in that a bulky, external control system 41 causes actuator shaft 39 to translate axially. The tapered surfaces of the actuator shaft 39 cause auxiliary cam elements 30,31 to protrude to change the profile of the cam to change.
Rosa (U.S. Pat. No. 4,388,897) discloses a system in which an axially movable wormed camshaft 1 rigidly supports primary cams 2 and rotatably supports secondary cams 3. Axial movement of the camshaft 1 causes both lateral and angular separation of the primary cams 2 with respect to the secondary cams 3.
Nelson et al. (U.S. Pat. No. 4,771,742) disclose a camshaft system similar to that of Woydt et al. and Goodfellow et al. Separation of camlobes 28,36 is electronically controlled with one or more microprocessors within a control unit 84. The primary disadvantage of Nelson et al. with respect to the present invention is the complexity, and hence expense, of the control arrangement.
Gondek (U.S. Pat. No. 5,090,366) discloses a system for splitting a pair of cams which control the hydraulic operation of a plurality of valves. However, the cam of Gondek indirectly actuates poppet valves of an ICE.
Norris (U.S. Pat. No. 5,178,105) discloses an arrangement including a cam follower and two camshafts to variably actuate a poppet valve. There is no suggestion of rotationally separating a pair of cams on a single camshaft to mutually operate a poppet valve.